Resources extracted from Heroes III.

I think there should be a video (gif) like the one in the window when you view monster info.

Now I understand what you meant, but still – there is no such video in the default resources of Heroes.

For example, let's look at the same unicorn in the castle fort.
As you can see, the unicorn's animation is quite long.
But throughout the video, there is a group of frames where the unicorn is simply walking (stepping), and the same frames of the unicorn walking are repeated on the screen.

Therefore, it would not be efficient to store an animation in which the same group of frames is repeated dozens of times, thereby greatly increasing the size of the resources.

It is easier to provide for a programmatic access to these frames and loop them dozens of times. In addition, this allows for randomizing the movement, i.e., literally creating a new (unique) video based on the available frames.

Here, the unicorn walks, then attacks, then walks again, then steps. And it can be done in any other order.

That's why these resources were created – to allow you to manipulate them in any order and save physical disk space ;)

And, by the way, the example of the unicorn's movement animation confirms all this.
The resources contained unique unicorn movements (a total of 63 frames).
And so, if you want to create an animation of the unicorn walking, you take all these individual frames and, by grouping and copying frames, create the unicorn walk that you want (you decide whether the unicorn will fall, whether it will take 10 steps or 5, etc.). As far as I understand, the same principle is used in Heroes.